The generation of cold is useful for many purposes. For example, it is employed in household refrigerators, air conditioning systems, industrial plants and the like, can be used to liquefy gases for rectification or storage and can be used to operate many devices which are more efficient at cryogenic temperatures. Such devices include, for example, superconductive electrical cables, cryogenic comminuting devices which embrittle a material before the milling thereof or the like. In addition cold-generating machines can be employed to maintain materials at low temperatures to protect them against deterioration or as part of a treatment, e.g. lyophilization or freeze drying, to preserve them.
The most common system in widespread use for the generation of cold is a refrigerant cycle which generally comprises a compressor, a cooler or condenser for the refrigerant downstream of the compressor, an expansion valve or throttle downstream of the cooler or condenser and a heat exchanger or evaporator in which heat is abstracted from the refrigerant. The heat exchanger or evaporator is connected to the input side of the compressor.
In an entirely different principle of operation, the compressed refrigerant can be cooled by permitting it to expand with so-called work-producing expansion, i.e. expansion in an expansion turbine against a load.
In throttled expansion, in which cooling occurs only when the gas to be expanded is cooled below its inversion temperature prior to expansion, the enthalpy during expansion is constant. The cooling is, therefore, the result of an increase in the potential energy of the gas molecules at the expense of their kinetic energy without the generation of cold in the sense of an enthalpy decrease. The resulting temperature reduction is, for a given pressure differential across the throttle, only relatively small.
A greater temperature drop is obtained with work expansion of a gas stream at the same pressure, the work expansion preferably being carried out nearly isentropically in the expansion machine. Thus not only is kinetic energy converted to potential energy, but additional cold is generated by the transformation of kinetic energy into mechanical work. The temperature reduction is here a function of the enthalpy difference in the gas stream before and after expansion.
In cold technology, especially in the technology of generating very low temperatures, e.g. temperatures at cryogenic levels, both thermodynamic principles have been used in the Linde process and in the process developed by Georges Claude. Both processes are substantially equivalent when high pressures and low temperatures are involved.
In the Linde process, the generation of low temperatures is effected exclusively by isenthalpic expansion of the gas stream to be cooled whereby the expanded gas stream is used to precool the not yet expanded gas. The advantage of the Linde process lies in the simple structure of the apparatus with which it can be practiced since, aside from the circulating compressor, no machinery with moving parts is necessary. The disadvantage, however, lies thermodynamically in the relatively low cold generation per circulated standard cubic meter of refrigerant because the cooling is a function exclusively of the isenthalpic, i.e. irreversible, expansion of the gas stream to be cooled.
In the Claude process, a partial stream of the gas to be cooled is work-expanded in an expansion machine, e.g. an expansion turbine, and is used to precool the remainder of the gas stream to be cooled. The latter gas stream is then expanded isenthalpically to the end pressure of the refrigerant-circulating path. The advantage of this process resides in the relatively deep cooling which can be obtained in the expansion machine by the work expansion of the partial stream. A disadvantage, however, lies in the need for the expansion machine, i.e. an additional machine with moving parts.
It should be noted, in connection with the use of expansion machines such as expansion turbines, that it is generally not possible to expand therein a gas phase which contains liquid droplets or a gas phase which may develop liquid droplets upon cooling within the expansion machine, since the presence of such droplets can damage the vanes and other parts of the turbine.